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I have a phrase I use often in my work, but I'm not sure if I'm hyphenating it correctly. As it stands, I've been writing it like this: The PCI-compliant payment gateway...

I think PCI-compliant is a phrasal adjective in the above example, so I'm writing the sentence as analogous to: The razor-sharp wire... (I may be wrong, though!)

However, I'm confused about whether or not to hyphenate in this case: The payment gateway is PCI compliant.

Should PCI compliant be hyphenated in this second case or not? Is it a phrasal adjective here?

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I just checked the Chicago Manual of Style (you can see my original answer from before that below) and found out some new things. CMOS 17 actually gives the general advice that "noun + adjective" should be hyphenated in attributive but "usually open" in predicative position, which isn't entirely intuitive to me. The examples given are "computer-literate accounts", "HIV-positive men", "the stadium is fan friendly" and "she is HIV positive".

The section about specific terms doesn't cover terms ending in "compliant", so as far as I can tell CMOS would recommend writing "The payment gateway is PCI compliant".


English hyphenation is complicated and often depends on "house style".

It is an adjective phrase in either context. Adjective phrases can occur in various positions: attributively, at the start of a noun phrase, or predicatively, after a copular verb.

Your uncertainly is probably related to the common rule about hyphenating "phrasal adjectives" in attributive position, but not in predicative position. This is mentioned in the answers to the following questions: "object-oriented" vs "object oriented", Do I keep myself "up-to-date" or "up to date" on something?

I think this rule originates from the fact that a phrase in attributive position is often pronounced as a compound word, even if it is not a compound in other contexts. For example, "in your face" is not normally a compound word, but in the expression "an in-your-face attitude", we pronounce "in-your-face" as a compound.

My own intuition is that "PCI-compliant" is a single, compound word in either position, which I think I can hear in the way it is stressed: for me the main accent is on the syllable "I" no matter where the phrase occurs (in contrast, the main accent for me in the phrase "up to date", which I will assume is not a compound word when it is not hyphenated, is on "date"). Therefore, I think I would hyphenate "PCI-compliant" in either position. However, compound words in English can be written with spaces (e.g. White House, which has the same stress pattern as blackbird or blue-eyed) so the stress pattern isn't necessarily a good piece of evidence for using a hyphen in "PCI-compliant".

I think may be relevant that the grammatical structure of "PCI-compliant" is noun + adjective rather than adverb + adjective. Compare words like moth-eaten and dog-eared, which as far as I know are often hyphenated in all positions.


Actually, don't compare it to "dog-eared" because I have realized that they have different structures. "Dog-eared" is [dog ear] + ed: it means that something has dog ears, just like something "winged" has wings, or someone "good-natured" has a good nature ([good nature] + -ed). Expressions like this (derived from an "adjective noun" NP by adding the suffix ed) seem to have a greater tendency than noun + adjective compounds to be written with hyphens.

However, the CMOS seems to recommend against hyphens even for these. In the section "adjective + participle", it says "Hyphenated before but not after a noun", and gives, among other examples, "tight-lipped person", "open-ended question" and "the question was open ended".

I think CMOS 17's preference for writing "was open ended" is a bit strange, and the Google Ngram Viewer and a quick skim of Google Books seems to suggest that it is not as common as "was open-ended":

"was open - ended" above "was open ended"

  • You're right, my initial confusion was due to attributive vs. predicative position for phrasal adjectives. But I hadn't even considered that it would be a compound word similar to moth-eaten or dog-eared. Those are helpful comparisons, thank you! – ennbee Feb 13 '18 at 23:36
  • @ennbee: glad I could help! As you may have picked up, I'm not entirely confident in this answer, so I'd recommend waiting to see what other people have to say. Hyphenation is a topic that I haven't studied in depth yet, so I could be wrong. – sumelic Feb 13 '18 at 23:38
  • I agree that "PCI compliant" is a single word (with or without the hyphen). But then I would say the same if a phrase stood in place of PCI, like "1935 version of the PCI specification-compliant" (example invented). – Greg Lee Feb 13 '18 at 23:43
  • @GregLee: Yes, that seems correct, although that raises the question of whether the first part should also be hyphenated in that context ("1935-version-of-the-PCI-specification-compliant"). Edwin Ashworth's answer to the following question suggests that it should be ("Compounds involving compounds") but my impression is that most people would prefer to write "White House-related" instead of "White-House-related", so I'm not really sure. – sumelic Feb 13 '18 at 23:50
  • Cerberus's answer to Hyphenating a compound noun in conjunction with '-related' just suggests rephrasing when a compound word would be excessively long. – sumelic Feb 13 '18 at 23:53

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